1. Field of the Invention
The present invention relates to transmissometers for measuring the opacity of fluid samples. In particular embodiments the invention relates to a transmissometer incorporating electronic shutters and an electronic stepwise light attenuator which are selectively actuated to place the transmissometer in the several light transmission modes necessary for opacity measurement, calibration and correction for drift.
2. Description of the Prior Art
Environmental considerations have given rise to regulatory requirements for the continuous monitoring of the opacity of smokestack effluents. The most reliable systems for providing this continuous monitoring are know as double-pass transmissometers. These devices include a light source on one side of the smokestack and an aligned retroreflector on the other side. A collimated light beam passes through the smokestack, is reflected by the retroreflector so that it passes through the smokestack again and then is picked up at selected time intervals by a photodetector whose output signals are representative of the opacity of the effluent through application of Beer's Law.
It is well known that transmissometers must be calibrated, preferably by calibration to a zero attenuation reference and upscale attenuation references. These conditions are achieved by directing a reference light beam from the source to the photodetector without allowing the beam to enter the smokestack.
A further consideration is that the transmissometer must be provided with means for compensating for "drift" caused by variations in the intensity of the light source, changes in the sensitivity of the photodetector, temperature changes, etc.
In order to deal with the calibration and drift problems, devices of the prior art have incorporated mechanical means for diverting optical paths and attenuating light in the optical path. These mechanical aspects of prior devices have proven quite complicated, and provide an aspect of unreliability that is undesirable in transmissometers which are usually placed at remote locations.